We have studied the binding efficiency of typical ligands (CO, NO) with ultrafast transient spectroscopy in mutants of myoglobin. These mutants have substituted amino acids in the neighborhood of the distal heme pocket. Ultrafast transient spectroscopy has been used to investigate both geminate and diffusional recombination of nitric oxide to mutant proteins. In addition, both Fe-heme and Co-heme substituents were also investigated in these studies.